In a portable system for ultrasonic medical imaging and diagnosing having CWD and PWD functions, CWD and PWD receiving circuits are so sensitive to noise generated by a power supply such that CWD and PWD images are susceptible to interference, so noise is a critical and difficult problem to the portable system for ultrasonic medical imaging and diagnosing. Generally, in a very small space of the portable system for ultrasonic medical imaging and diagnosing, to improve an efficiency of the power supply, an efficient switching DC converter, i.e. a DC-DC converter, is used to realize a power conversion, for example, a reduction from a high voltage (e.g. 20V) to a voltage (e.g. 5V) that is actually needed by a circuit, in order to supply the needed voltage to the circuit of the system. However, the switching DC-DC converter will generate some radiation interference noises (e.g. interference and noise generated by harmonics) while improving the efficiency of the power supply, thereby influencing image quality in the CWD and PWD receiving circuits. The portable system for ultrasonic medical imaging and diagnosing in the prior art either gives no consideration to the influence of the noise of the DC-DC converter on the CWD and PWD images or fails to put forward a good solution to such influence.